Polyester resins are widely used on account of their various excellent characteristics. However, they have low dyeability and can hardly be dyed except with disperse dyes. Among various proposals to improve the dyeability of polyester resins, one is copolymerization of an isophthalic acid component containing a sulfonic acid salt with a resin to endow the polyester with dyeability with cationic dyes, as disclosed, e.g., in JP-B-34-10497 (the term "JP-B" as used herein means an "examined published Japanese patent application").
However, such a polymerization system has considerably increased a melt viscosity of resins due to the thickening effect of the sulfonate-containing isophthalic acid component and thus has low moldability because of an increase in spinning pressure and a reduction in spinnability. Therefore, thus obtained polyester fibers tend to be deteriorated in strength due to a decreased polymerization degree and have found limited use and have not been useful alone in sportswear, paraglider wings, sails of yachts, suits, etc.
Increasing the melting point is one approach to reduce melt viscosity but increasing the melting point accelerates resin degradation. As a result, the degree of polymerization is decreased, so that one fails to obtain a cationic dye-dyeable polyester fiber having a high polymerization degree and high strength. In order to overcome these problems, the addition of a lubricant has been suggested. However, the addition of ethylenebisstearamide, stearic acid or stearyl alcohol as a lubricant to a resin compound decreases the melt viscosity and also decreases the degree of polymerization of the resin.
On the other hand, it has been proposed to add melt viscosity depressants to polyester resins, to thereby obtain high strength polyester fibers, e.g., in W091/02111. However, cationic dye-dyeable polyester resins are specific polymers having a very high viscosity as compared with the resins to which the melt viscosity depressant has been added conventionally.
Under such a circumstance, the development of cationic dye-dyeable polyester resin has been desired in which the melt viscosity is reduced while cationic dye-dyeability is maintained.